The present invention relates generally to antennas, and more particularly to broadband micropatch antenna systems with reduced sensitivity to multipath reception.
Micropatch antennas (MPAs) are widely deployed in global navigation satellite system (GNSS) receivers. Relative to other antenna designs, they are small and lightweight, and they may be manufactured in high volumes at low cost. The basic elements of a conventional MPA are a flat radiating element (patch) and a flat ground plane separated by a dielectric medium. The resonant size of a MPA is a function of the wavelength of the radiation propagating in the dielectric medium between the radiating element and the ground plane. The resonant size is approximately half of the wavelength. The resonant size may be reduced by increasing the permittivity of the dielectric medium or by introducing wave-slowing structures. Reducing the resonant size also yields a wider antenna pattern, which is advantageous for some applications.
The size of an MPA is also determined by other design considerations. In conventional MPAs, the size of the ground plane is typically greater than or equal to λ, where λ is the free-space wavelength of the radiation of interest. A large ground plane is used to reduce signals reflected from the terrain below the antenna. Furthermore, in an MPA, the bandwidth increases with the height of the radiating element above the ground plane. To achieve a bandwidth of 12% or more, the height is ˜(0.10-0.15)λ. Here, the bandwidth is specified as percentage of the central frequency corresponding to λ. In addition to increasing the overall size of the antenna, however, the required height also results in an increased radiation pattern in the backward hemisphere and higher sensitivity to multipath reception. High sensitivity to multipath reception becomes significant when the length of the ground plane is on the order of 1-1.5 wavelengths.
FIG. 1 shows one prior-art micropatch antenna design for reducing sensitivity to multipath reception. The micropatch antenna is a microstrip antenna formed on dielectric substrate 110. Positioned on top of dielectric substrate 110 are patch antenna elements 104. Positioned underneath the dielectric substrate 110 is a ground plane 102, which includes edge ground elements 108. Coaxial feed-point 106 connects to the patch antenna elements 104. A short-maker (not shown) connects together the patch antenna elements 104 with the ground plane 102.
Edge ground elements 108 form a vertical rim near the edge of the dielectric substrate 110 and project above the top surface of dielectric substrate 110. In one embodiment, parts of edge ground elements 108 include conducting through holes located near the edge of the dielectric substrate 110. The spacing between the holes is a defined value much smaller than a wavelength. Presumably, the design of edge ground elements 108 causes substantial filtration of multipath radiation propagated from below the horizon. This prior-art micropatch antenna design, however, suffers a major disadvantage: reduction in sensitivity to multipath reflection is inefficient in the case of a broadband radiator with lengths of the ground plane on the order of 1-1.5 wavelengths. Higher bandwidth is achieved by increasing the height of patch antenna elements 104 above ground plane 102, resulting in higher sensitivity to multipath reflection.
What is needed is a broadband micropatch antenna system with small size, wide bandwidth, and low sensitivity to multipath reception.